This invention relates to techniques for generating and detecting ultrasonic waves.
Efficiency and economy have been increasingly emphasized in many aspects of modern structural design, and this emphasis has stimulated the more widespread use of nondestructive testing techniques. Before nondestructive methods were developed, it was necessary to assume, in designing structural components, that flaws of a certain size were present in the construction materials. This design technique required the selection of structural components which were of sufficient size and adequate in strength to withstand failure even if the assumed defects were present. The introduction of nondestructive testing methods, however, has made feasible the identification of a structural defect at an early stage in the life of the underlying flaw, so that the appropriate corrective action, such as removing and replacing a defective component, can be accomplished before the defect causes a catastrophic failure. Consequently, where nondestructive testing can be implemented throughout their operational lives, structural components may be manufactured and assembled more economically by reducing dimensions and substituting lower strength, less expensive materials. Nondestructive inspection techniques can thus be utilized to maintain a desired level of reliability in a structure while concurrently reducing construction and materials costs. Nondestructive testing can similarly be used to extend the service life of existing structures.
One of the most useful types of nondestructive testing involves ultrasonics, in which the interactions between acoustic wave energy and the internal structure of an object are analyzed to predict the physical integrity of the object. A key element in an ultransonic nondestructive testing system is the transducers, which are used to convert electrical energy into acoustic wave energy in the test object and also to convert the acoustic energy back into electrical energy for detection purposes. Traditionally, the high conversion efficiency and modest cost of piezoelectric materials have influenced their widespread use as ultrasonic transducers in many applications. Piezoelectric transducers are disadvantaged, however, by their need to be coupled to the ultrasonic medium through a liquid or solid bond.
Because of these limitations to the piezoelectric approach, applications with demanding performance requirements, such as, for example, operation at high speeds, at elevated temperatures, in remote locations, with broadband and reproducible acoustic coupling, and without the subsequent cleanup of a liquid bond, have spurred the development of noncontact ultrasonic methods, such as electrostatic tranducers, optical techniques, and electromagnetic transducers. These techniques have supplanted piezoelectric transducers in many applications. One of the most promising noncontact transducers is the electromagnetic acoustic transducer (EMAT). An EMAT consists of an electrically conductive coil which is positioned within a static magnetic field extending into the surface of a conducting material. When a radio frequency signal is applied to the coil, eddy currents are induced in the material. If the magnetic field and the coil are properly oriented, the Lorentz forces which are exerted on the eddy currents by the magnetic field will be transferred to the lattice structure of the material and thereby generate an ultrasonic wave. Reduced inspection time, an ability to operate in remote and inaccessible locations, and diminished transducer wear are some of the significant advantages offered by an EMAT-based nondestructive testing system.
EMATs have been fabricated with a variety of coil and magnet configurations to suit the requirements of particular applications. U.S. Pat. Nos. 3,850,028; 4,048,847; 4,080,836; 4,092,868; 4,104,922; 4,127,035; 4,184,374; 4,218,924; 4,232,557; 4,248,092; 4,295,214; and 4,296,486, for example, the teachings of which are incorporated herein by reference, illustrate some of the approaches which have been utilized.
EMATs have been employed to great advantage in a number of nondestructive testing situations, and ongoing research is continuing to identify additional applications for these devices. One of the goals of this research is to improve the signal to noise ratio of EMAT systems by increasing the amplitude of the acoustic energy which can be generated by an EMAT and by increasing the signal level produced by an EMAT in detecting acoustic energy at a given amplitude. In particular, a need has developed for a practical system which would enable an EMAT to generate a highly directional beam of acoustic energy or selectively detect only that acoustic energy propagating in a particular direction.